Embodiments of the invention described in this specification relate generally to measuring fluid flow rate, and more particularly, to an outflow (or “flow out”) measurement system that determines fluid flow by volume and percentage measurements and retains consistent high-sensitivity and high-accuracy performance throughout a measurement range of zero flow to maximum flow utilizing a method for combining fluid level measurement obtained at the outflow vertical section of a drilling rig (i.e., bell nipple or post annular blowout preventer (BOP) area) and flow measurement obtained at the partially-filled pipe section (i.e., return flow line).
Drilling rigs are typically designed with an enclosed or open-channel return flow line. As fluid flows through this return flow line section, it partially fills up the flow channel or pipe, as opposed to filling up the whole pipe like in a full pipe flow regime. Flow indicators or sensors installed on the partially-filled pipe section have poor sensitivity performances especially as flow approaches low levels such as during pumps-off conditions (i.e., when rig pumps are turned off) wherein flow approaches zero flow. Additionally, subtle increases or decreases of flow (e.g., at the onset of a kick or a loss condition) are largely dissipated throughout the length of the return flow line making the subtle change difficult to be detected if only using information from sensors mounted on the return flow line.
Today, existing flow out measurement systems are typically only flow line mounted and in this location, the sensitivity to changes in fluid flow behavior is naturally impaired due to dissipation of subtle changes of flow throughout the return flow line. Minor changes to flow possibly caused by fluid influxes from the formation or fluid losses to the formation, and the back flow (or “flow back”) fingerprints created during pumps-off conditions are not sufficiently detectable throughout the return flow line.
Therefore, what is needed is a way to address the unsatisfactory measurement performance of typical existing flow out measurement systems.